Single or multiple brushless DC cooling fans are widely employed in AC-DC, DC-DC and DC-AC power conversion systems to remove the heat generated by semiconductor switching devices, various magnetic components and other circuit components that are part of the power conversion equipment. Typically, the cooling fan allows the power conversion equipment to be operated at higher temperatures. As a result, the proper operation of the cooling fan plays an important role in overall power system reliability and lifetime.
In most applications, a speed feedback signal from the fan is employed to govern a fan alarm or a system-wide safety interlock. Whenever the speed feedback signal is not detected, the alarm is tripped or the power conversion system is shut down to protect "on-board" circuit components of the power conversion system from excessive heating. In the normal course of assembly and operation, a fan alarm and possible shutdown may be caused by one or all of the following: a broken fan power cable(s), a shorted fan power cable, a loose fan cable connection, an incorrect wiring of the fan power cable, a malfunction of the fan's internal speed sensor, a problem with the fan's power supply and a malfunction of the fan's speed detection circuit.
Generally, after the fan has been assembled into the power conversion system, it becomes difficult (and sometimes impossible) to inspect the fan visually to determine whether it is operating properly or to identify the nature of a malfunction. Alternative, non-visual inspection methods typically require complex inspection/detection circuitry that increase the fan's material and manufacturing costs. The primary reason for the complexity of the inspection/detection circuitry is that the fan speed feedback signal can either be a variable frequency alternating signal, e.g., a 60 Hz to 120 Hz pulse train, or a constant voltage signal, e.g., a 0V or 5V signal.
The form of the fan feedback signal depends on the status of the fan. In the case of a brushless DC fan, the fan feedback signal assumes a constant voltage when the fan is stalled or not running. During normal operation, a pulse train with a peak amplitude of e.g., 5V, is provided at the fan's feedback signal terminal.
The different forms and signal levels that might be encountered results in existing on-board fan detection circuits that are quite cumbersome and complex. Accordingly, what is needed in the art is an improved fan operation detection circuit that overcomes the above-described limitations.